Automatic frequency control apparatus



y 25, 1953 H. L. SCHULTZ 2,640,156

' AUTOMATIC FREQUENCY CONTROL APPARATUS Filed Oct. 31, 1945 2 Sheets-Sheet 1 .363? ll-- ggafi'w DETECTOR A BTRANSFORMER AMPUFIER 1 I I I 7 L MIXER (I3 25 26 T 6 l 3| l A A AUDIO 6 BLQCNNG GAS TUBE i OSCILLATOR OSCILLATOR I I I Q0 (44 4 29 FIG. 2

CAVITY RESPONSE I y INVENTOR HOWARD L. SCHULTZ.

' FREQUENCY l5 v (:BQIZIIS ATTORNEY y 26, 1953 I H. L. SCHULTZ ,640, 56

AUTOMATIC FREQUENCY CONTROL APPARATUS Filed Oct. 31, 1945 2 SheetS-Shee t 2 NEGATIVE REFLECTIVE vou's SEARCH IN PHASE CAVITY RESPONSE INVENTOR HOWARD L. SCHULTZ 3T EM ATTORNEY Patented May 26, 1953 AUTOMATIC FREQUENCY CONTROL APPARATUS Howard L. Schultz, Wellesley, Mass, assignor, by mesne assignments, to the United States of America as represented by the Secretary 01: the

Navy

Application October 31, 1945, Serial No. 625,883

Claims. 1

My present invention relates in general to automatic frequency control of radio equipment, and in particular to a radio receiver intended for the reception of radio waves of one frequency only, such as for example, a radio beacon receiver.

"Since radio beacon transmission is carried on usually at a single frequency, it is desirable that equipment intended for use in such a system transmit on and respond to that single frequency to a high degree of accuracy. Accordingly, such equipment may often be equipped with a circuit for maintaining its frequency substantially constant. usually called an automatic frequency con trol (A. F. C.) circuit. This need for frequency stabilization of a beacon system becomes greatly important at the higher frequencies. My invention is accordingly illustrated hereinafter as embodied in a radio receiver designed for the reception of energy of such higher frequencies, it being understood, however, that this .is only an example of the manner in which the invention can he used.

Briefly, the invention comprises an oscillator of the reflex cavity type in which the frequency is controlled by a negative potential applied to the reflector electrode, and a tuned cavity which is tuned to the frequency at which the oscillator is to be stabilized. An audio signal modulates the oscillator signal and a detector fed by the referonce cavity. reproduces this audio signal in one of two opposing phases depending upon whether the oscillator signal has a frequency above or below the resonance frequency of the cavity. A single gaseous electron tube in a coincidence circuit controls the potential on the reflector electrode in accordance with the phase of the audio signal produced by the detector.

It is a primary object oi? my invention to provide automatic frequency control for a radio system' will maintain the frequency of an oscillator substantially constant with the use of a minimum number of electron tubes,

It is another object of my invention to provide a coincidence control circuit that may he con structed of asingle electron tube.

Other and further objects and features of my invention will become apparent upon a careful consideration of the following detailed description when taken together with the accompanying drawing, the figures of which illustrate a typical embodiment of the invention.

In the drawing:

Fig. 1 is a block diagram of an automatic frequency control circuit in accordance with my invention;

Fig. 2 is a diagram partially in block and partially in electrical scheme of a circuit in accordance with Fig. 1;

Fig. 3 illustrates the action of the reference cavity and rectifier of Fig. 1;

Fig. 4 illustrates the action of the search circuit of Fig. l with respect to the reference cavity response;

Fig. 5 illustrates the voltage graph with respect to time of the potential applied to the reflector electrode of the local oscillator of Fig. l; and

Fig. 6 illustrates phase relationship among various voltage waves existing in the apparatus of Fig. 1.

In Figs. 1 and 2, a local oscillator Ill energizes a mixer H to which conventional R, F. signals may he brought and from which I. F. signals may be taken, which in addition energizes a tuned reference cavity 42 which is fixedly tuned to a predetermined desired local oscillator frequency. A detector 13 rectifies the local oscillator signal in a conventional manner. An audio oscillator M. which way oscillate at a rate from 800 to .1600 cycles per second, energizes the local oscillator Ill in such a manner as to modulate the local oscillator signal. The detector [3 produces a signal H5 or H having the same frequency as the signal 15 from the audio oscillator It and the same or the opposite phase respectively, depending upon the frequency of the local oscillator ill with respect to the resonance frequency of the reference cavity l2, as will now be explained with reference to Fig. 3.

In Fig 3, the local oscillator frequency is represented by a dotted line B-B, C--C' or G-G', each of these lines representing a particular operating frequency 12, h, or in respectively of the cavity 42. The resonance frequency is fo, and a frequency above or below that frequency is In orji, respectively. The response curve 20 of the cavity 12 is a conventional tuned cavity response curve. When the local oscillator H! is oscillating at .a frequency f1 below the resonance frequency ft of the cavity 12, the audio sine wave l5 illustrated on the line C--C will be reproduced in like phase in the output of the detector 13 as another audio sine wave 1'6, line 0-0, since the slope of the cavity response curve 20 is positive at the operating point 2| for an operating frequency ,ifi below the resonance frequency f0. When the operating frequency is of the local oscillator to is above the resonance frequency in of the cavity'lz, as illustrated along the line 3-43, the output of the detector 1'3 is a wave l1 similar to the input sine wave 15, but oppositely phased, as illustrated on the line B-B, the response curve 20 of the cavity [2 now having a negative slope at the operating point 22. In the case where the frequency of the local oscillator I is identical with the resonance frequency in of the cavity E2, the input sine wave l5 results in a double frequency output sine wave l8, illustrated along the line GG, the slope of the response curve being alternately positive and negative about the operating point 23. This is a critical condition, however, and in operation one or the other of the two sine waves IE or I1 is alternately provided by the detector I3.

The detector output signal 16 or I1 is amplified in a transformer and further amplified in an electron tube amplifier 26, comprising an electron tube 46 and suitable associated circuit components. The transformer 25 is used to permit the elimination of an electron tube, and it is to be understood that an ordinary electron tube amplifier may be used in place of this transformer, if desired. The output signal of the amplifier 26, corresponding to the signal I6 or I1 from the detector I3, is fed to a gas tub coincidence circuit 21, comprising preferably a gaseous electron tube 28, at the input terminal 29 thereof, leading to a first grid 36. Simultaneously, a sine wave [5 from the audio oscillator I4 is brought to the coincidence. circuit 21 through a second input terminal 3! leading to a second grid 32 of the electron tube 28. The electron tube 28 has its anode 33 grounded through a series pair of resistors 34 and 35. The cathode 35 is connected to a source of negative voltage, which may have a value in the neighborhood of -300 volts, through suitable series resistors 31 and 38. Negative bias is provided for the first grid through a resistor 39 connected at one end to the junction of the last-mentioned resistors 31 and 38. A capacitor 4| is connected from the junction of the resistors 34 and in the anode circuit to the cathode 36. The output 63 from the coincidence circuit 21 is taken at the junction point 42 between the side of the capacitor 4I nearest ground and the resistor 35 nearest ground in the anode circuit. This output 63 of the coincidence circuit 21 is fed through a resistor 43 and a blocking oscillator circuit 44, comprising an electron tube and other components to the reflector electrode 5|] of the local oscillator l0 and controls the potential on that electrode 50.

The blocking oscillator 44 comprises the electron tube 45 having an anode 41, a cathode 48, and a grid 49, and a transformer 5| having primary 60 and secondary 6| windings connected in the usual fashion to provide blocking oscillator action, and to a source of positive voltage, which may be in the neighborhood of +105 volts. The output of the blocking oscillator 44 is taken from the grid 49 through the secondary winding 5| at a terminal 52. the oscillator I4 is brought through a suitable coupling capacitor 52 to the cathode 48 of the electron tube 45, and then through a second capacitor 53 to the reflector of the local oscillator ID. A potentiometer 55 having a movable arm 56 which is connected to the cathode 48 of the electron tube 45 is inserted in a suitable resistor path between the negative voltage source 300 Volts and ground and provides cathode bia for the blocking oscillator electron tube 45. A capacitor 54 is provided between the cathode 48 and ground and is associated in series with the adjacent capacitor 53 and the coincidence circuit output resistor 43 in an integrator network. that The audio signal I5 provided by smoothes the output wave 53 of the coincidence circuit 21 into the final reflector voltage wave 64.

The operation of the circuit of Figs. 1 and 2 is as follows. When the circuit is turned on, positive potential is brought to the anode 41 of the blocking oscillator tube 45 through the primary 60 of the transformer 5|. The secondary SI of the transformer 51 is connected at one end to the grid 49 in such fashion as to apply increasingly positive potential to the grid 49 as the conductivity of the electron tube 45 increases in the usual fashion of a blocking oscillator circuit. The opposite end of the secondary 6| is connected to the reflector 50 of the local oscillator Ill. Thus the grid voltage wave of the blocking oscillator 44 is available to the reflector electrode 50. When the blocking oscillator 44 has com pleted one pulse of its oscillation and the tube 45 has ceased to conduct, the voltage of the grid 49 will substantially instantaneously drop to a highly negative value E1,' from which highly negative value it will rise slowly toward ground potential or as illustrated in Fig. 5 toward the;

value -Ez, where a second pulse of oscillation may take place. through an intermediate or quiescent value Eo. which is the required negative value of reflector electrode potential for maintaining the local oscillator ill at the desired frequency. This grid voltage wave would gradually rise from the lower negative value -E1 toward the lesser negative value E2 only if the blocking oscillator 44 were permitted to oscillate freely in its natural fashion, having a somewhat saw-toothedform, as shown in dotted lines after the value -E3. action of the blocking oscillator 44 is interrupted, however, at a value Ea slightly less negative than the desired negative value Eo, as will hereinafter be explained.

While the potential on the grid 49 i highly negative, E1, the local oscillator ID will be oscillating at a frequency I: which is higher than the resonance frequency in of the cavity [2. 'As

illustrated in Fig. 3 and hereinabove explained,-

when the local oscillator I0 is oscillating at this higher frequency f2, the output l1 from the detector [3 will be phased oppositely to th sine wave 15. For this reason, the signals on the two grids 30' and 32 of the electron tube 28 in the coincidence circuit 21 will have opposite phases. In Fig. 4, the shaded area under the curve 20 represents the region of such phase opposition.

The coincidence circuit 21 is so arranged that the signals on the grid 30 and 32 must be in phase before the electron tube 28 may conduct current, as'shown in Fig. 6. As the potential on the re fiector electrode 50 rises toward the less negative value Ez, corresponding to a search toward lower frequencies in Fig. 4, the frequency of the local oscillator I0 drops, passing through the peak of the responsefrequency curve "of the cavity l2,

less negative value Ea, the detector output 16' which is in phase with the audio sine wave [5 is had. This is represented by the unshaded area under the curve 20 in Fig. 4. At this time the signals on the grids 30 and 32 of the electron tube 28 in the coincidence circuit 21 are in phase and. Since the.

the tube 28 begins to conduct current.

The grid voltage wave passes.

The

MMOAQB tube 2 8 is a gaseous electron tube, conduction will be substantially instantaneous and the impedance provided by the tube 28 in the circuit will substantially vanish. As a consequence, a large current flows in the resistors 34 and 35, and the resistors 3'7 and 38, due to the presence of the highly negative potential, 300 volts, applied through those resistors and the electron tube 28 to ground. The voltage on the anode 33 then suddenly substantially instantaneously drops from ground potential to a negative value b in its output wave 63, shown clearly in Fig. 6. This drop in pot-ential is fed through the resistor' 4-3 to the grid 49 of the blocking oscillator tube 45, thereby interrupting the rise in potential of the grid 49 at the voltage -E3 and lowering the potential on the reflector electrode 50 or the local oscillator Hi. This reverses the direction of search in Fig. 4, and the frequency of the local oscillator is increased to the point where it is now greater than the resonance frequency in of the reference cavity i2. As a further consequence, the output I! of the detector 13 is now oppositely phased to the sine wave 15 from the oscillator i4. Thus, the signals on the grids 3!) and 32 of the electron tube 28 in the coincidence circuit are now oppositely phased.

The electron tube 28 is substantially self extinguishing inasmuch as the sudden drop in potential of the anode 33 will cause the tube to cease conducting current. Simultaneously with the drop in potential on the anode 33, the capacitor ll is charged negatively to the same extent, the side of that capacitor nearest ground taking on this new negative potential. After the elec tron tube 28 has ceased to conduct current, this capacitor 6! discharges through the thereunto connected resistor 35 to ground; Depending on the value of the resistor 35, this discharge takes a substantial amount-of time, and the voltage 63 at the output point 42 rises gradually to a value 0. Since this voltage is ultimately applied to the reflector 50 of the local oscillator H1, eventually lowering its frequency to the value f1 below the resonance frequency in of the cavity it, at this value 0 there is coincidence again between the phases of the signals on the grids ill and 32 of the coincidence circuit 21. This point 0 thus becomes the point a which the electron tube 23 will again begin to conduct and the process will repeat itself. There is thus provided a relatively sawtoothed. wave [53, shown along the axis 53-43 as the output of the coincidence circuit 27,.w'h icih wave as maintains the reflector voltage close to the quiescent value Eo.

This output $3 from the coincidence circuit .27 thus keeps the grid 49 of the blocking oscillator tube 55 from ever again reaching the less negative value -Ez at which the blocking oscillator l will again commence normal oscillation. Thus the blocking oscillator circuit 44 is valuable only to start the automatic frequency control circuit into operation, as illustrated in Fig. 5.

The output wave 53 of the coincidenc circuit 2i is somewhat sawtoothed, and therefore not sufficiently smooth to be used as an automatic frequency contro1 voltage for the reflector electrode fill. An integrator circuit, comprising the coincidence circuit output resistor 43 and two capacitors 53 and 54 in series to ground, smoothes this wave 63 so that at the output point 62 a wave 64 having a smoother shape is had, which is the control Wave applied to the reflector electrode 56. This wave oscillates about the quiescent reflector voltage -Eo at a frequency equal to the frequency 6. of the :sine wave output lofitheaudio osoillator H. rt is a substantially smooth wave, however, with small amplitude and maintains the ireuuency of the local oscillator t0 substantially stable. With "proper lo'alancing "of 'the resistor 33 and the capacitors '53 and 54 of the integrator network, the output wave 6! attheapo'inttz :may be made to have a substantially smooth form. Thezdownward :portion, 0 to -b, of the -output "wave 63 of the 'oonoidence circuit 2 charges the con-- the local oscillator tube L0 to oscillate at the desired frequency.

:As shown Fig. 6.,13115156 coincidence between the positive peaks of the sine wave 15 andthe inphase detector output wave t6 determines the time of conduction of the coincidence tube 28, and hence of the negative excursion a to b" of the output wave -63 of that tube. preferred adjustment, to assure that the tube 2 8 will become conductive only at this time.

As will be readily appreciated, although in a receiver the mixer :M will also have the usual received R. signals introduced into it, and the usual detector and I. F. amplifier of .a radio receiver energized by its normal output, in other applications my invention, the mixer 11 may be omitted. It is then apparent that there remains a circuit for maintaining automatic frequency control of any oscillator, as for example, that of a transmitter, or a test standard. Since this and other changes may be :made in the above described article and diiferent embodiments of the invention could be made without departing from the scope thereof, it is intended that all matter contained in the description hereinabove or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense, and that the invention is not to be limited except insofar as is necessitated by the prior art and the spirit of the appended claims.

What is claimed is: v

Automatic frequency control apparatus comprising, an oscillator circuit of the type whose frequency dependent upon the electric. potential applied to an electrode thereof adapted to oscillate substantially at a desired frequency, means for modulating the output of said circuit with a first oscillatory signal, a resonant cavity tuned to said desired frequency having as its input the output of said circuit, a detector having as its input the output of said cavity and as an output an oscillatory reference signal of like frequency to said first oscillatory signal, said reference signal having a first phase similar to that of said first oscillatory signal when the frequency of said osc1llator is below the resonance frequency of said cavity and a second opposite phase when said osc1llator frequency is above said resonance frequency, means for periodically varying the value of said electric potential between relatively broadly spaced limits including the value required to provide said desired frequency, and a coincidence is the circuit responsive to phase coincidence between said first and reference oscillatory signals for interrupting the action of-said last mentioned means at a time when said electric potential is substantially at said desired frequency value and thereafter maintaining said potential substantially at said desired frequency value.

2. Apparatus in accordance with claim 1 in which said coincidence circuit comprises a gaseous electron tube having at least two grids, an anode, and a cathode, the first of said grids having as its input the output of said detector, the second of said grids being energized by said first signal, said anode being grounded through a first resistor, and said cathode being connected through a second resistor to a source of negative voltage, and a capacitor connected from said anode to said cathode, the side of said capacitor nearest ground being further connected to said electrode of said oscillator.

3. Apparatus in accordance with claim 1 in which said coincidence circuit comprises a nor' mally non-conductive gaseous electron tube having at least two grids, the first of said grids having as its input the output of saiddetector, the second of said grids being energized by said first signal, said anode being grounded through a first resistor, and said cathode being connected through a second resistor to a source-of nega-' tive voltage, and a capacitor connected from said anode to said cathode and further connected to said electrode of said oscillator, said normally non-conductive gaseous electron tube becoming conductive when the signals on said two grids are in phase, whereby the anode of said gaseous tube becomes negative with respect to ground and said electrode simultaneously becoming lower in potential.

4. Automatic frequency control apparatus comprising, an oscillator circuit adapted to oscillate substantially at a desired frequency, means for modulating the output of said oscillator circuit with a first signal, a resonant circuit tuned to said desired frequency having as its input the output of said oscillator circuit, a-detector having as its input the output of said resonant circuit and as an output a reference signal of like frequency to said first signal, said reference signal having a first phase similar to that of said first signal when the frequency of said oscillator is below the resonant frequency of said tuned circuit and a second opposite phase when said oscillator frequency is above said resonant frequency, means for periodically varying the output frequency of said oscillator between broadly spaced limits including said desired frequency, and a coincidence circuit responsive to phase coincidence between said first and reference signals for interrupting the action of said last-mentioned means when said output frequency is substantially said desired frequency and thereafter maintaining said output frequency at said desired frequency.

5. Automatic frequency control apparatus comprising, a primary oscillator for providing a signal substantially at a desired frequency determined by voltage applied thereto, means for modulating the output of said primary oscillator with a signal of frequency substantially lower than said desired frequency, a cavity resonator fixedly tuned to said desired frequency, means for applying the modulated output of said primary oscillator to said cavity resonator, a detector for demodulating the output of said cavity resonator and providing a signal of a first phase in response to one direction of deviation of the output of said primary oscillator from said desired frequency and a signal of a second phase opposite to said first phase in response to deviation of the output of said primary oscillator from said desired frequency in a direction opposite to said first direction, a gas tube coincidence circuit having two input channels, means for applying the output of said detector to one of said input channels, means for applying a portion of the output of said modulating means to the other of said input channels, said gas tube coincidence circuit becoming conductive only in response to phase coincidence between the signals applied to said two input channels, a blocking oscillator for providing high frequency sweep voltages, which sweep voltages pass through the critical voltage required to maintain said primary oscillator at said desired frequency, means connecting said gas tube coincidence circuit to said blocking oscillator to interrupt said sweep voltages as they reach said critical voltage, and means connecting the output of said blocking oscillator to said primary oscillator as a frequency controlling voltage, the frequency of the output of said primary oscillator determining the times of conduction of said gas tube coincidence circuit and thereby the sweeping action of said blocking oscillator.

HOWARD L. SCHULTZ.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,261,800 Freeman Nov. 4, 1941 2,379,689 Crosby July 3, 1945 2,404,568 Dow July 23, 1946 2,406,125 Ziegler et a1 Aug. 20, 1946 2,434,294 Ginzton Jan. 13, 1948 2,462,294 Thompson Feb. 22, 1949 2,475,074 Bradley et al July 5, 1949 2,510,095 Frankel June 6, 1950 

